1. Field of the Invention
The present invention relates to a line terminating device such as an optical network unit (ONU).
2. Description of the Related Art
In recent years, a transmission line for a communication device has been on the way of replacement from an electrical cable (coaxial cable or twisted cable) into an optical cable. Particularly, it has been considered that a line terminating device related to fiber-to-the-home (FTTH) is to be introduced to every home, so that size reduction of the device has been demanded. From the viewpoint of size reduction of the device, a mounting structure exhibiting the maximum functions of mounted components occupies a considerable proportion of importance and it is therefore necessary to design a mounting structure suitable for the size reduction. Further, it is also necessary to design a structure for efficiently accommodating extra lengths of optical cables connected to the line terminating device.
Although the FTTH-related line terminating device is required to have a compact size in consideration of its introduction into every home, it can be said that a line terminating device in the related art does not enough meet the requirement for the size reduction. Further, while a subscriber optical cable and a terminal optical cable are connected to the line terminating device, there is room for improvement in workability of connection of the optical cables. An ATM (asynchronous transfer mode) private line system is known as the FTTH-related system. The ATM private line system is a system in which multimedia is handled as a main service and small and medium business users are targeted.
An ATM-ONU is connected through an optical subscriber line to an ATM-SLT (subscriber line terminal) to configure a subscriber loop system. The optical subscriber line is an optical transmission line with a transmission rate of 155.52 Mbit/s, for example. The ATM-ONU is installed in a user building and accommodates user terminal lines. By mounting a line card into the ATM-ONU, connection to various terminal equipments is allowed.
Although the conventional line terminating device is provided with an area for accommodating an extra length of the terminal optical cable, a terminal optical connector is mounted in the vicinity of an end surface of a housing of the terminal device. Accordingly, in connecting the terminal optical cable to an extension electronic circuit package, the optical cable projects from the housing, so that a protective cover is required. Further, the conventional line terminating device has a laser beam cutting mechanism for cutting off a laser beam outputted from an optical module to prevent a trouble on human eyes. However, this mechanism does not form fundamental means for preventing the trouble, but hinders size reduction and cost reduction of the device.
In the conventional line terminating device, a shield spring is used to prevent leakage of undue electromagnetic noise from the device, causing an increase in number of parts and accordingly a cost increase. Further, the extension electronic circuit package is undesirably withdrawn from the device even by not disconnecting the terminal optical connector from the device, causing a possibility of improper operation. Thus, the conventional line terminating device is not satisfactory in measures for preventing such improper operation.